Capacitor question

Thread Starter

lukimaister

Joined Sep 18, 2020
25
Hi,

I'm working on a little project and I have a question about capacitor. If 400V AC capacitor would be connected to 400V source, the capacitor could store around 565V DC (400V x √2), if I'm not wrong. So what if there would be over 500V DC source, would capacitor still be able to hold , because it can potencially be charged to 565V DC?

I have 400V AC capacitor and I want to connect it to around 500V DC source, or would it be safer to lower source to 400V DC?
 

bertus

Joined Apr 5, 2008
21,239
Hello,

What does the datasheet of the capacitor say about the AC and DC voltages?
I would not go over 400 Volts if there is nothing stated about the DC characteristics.

Bertus
 

Tonyr1084

Joined Sep 24, 2015
5,807
There's no rules that say a cap can not be overly oversized. If there's a potential voltage (AC or DC) of 566 volts I'd opt for a cap that can handle anywhere fro 33% to 50% over that voltage. In other words, 753V to 792V. As often many people do - double the rating. 566 becomes 1132 volts. A cap rated at 1KV should be a well safe choice. It's bigger, but it's also safer. The only reason why I would opt for a smaller cap would be when size constraints demand it. But then I have to decide whether it's a safe project to pursue.
 

MisterBill2

Joined Jan 23, 2018
8,214
AC capacitors are not rated for DC operation, and because most AC rated capacitors are for motor circuits, the DC leakage current may be more than you would want in a typical DC circuit. Many AC rated motor circuit capacitors have a resistor permanently across the terminals to quickly drain off any voltage stored when the power is switched off.
 

MaxHeadRoom

Joined Jul 18, 2013
22,641
Depends on what motor capacitors you are talking about, the Run style are usually oil filled paper and are just as happy on AC or DC, the Electrolytic AC start caps are not polarity marked and are constructed especially for AC start with solid electrolyte.
Max.
 
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